An explanation will first be given of a typical color liquid crystal display device with reference to FIG. 1 of the drawings attached hereto.
In FIG. 1, there is shown a pair of glass substrates 1A and 1B which are spaced apart from each other. Of them, the one glass plate 1A has, outside thereof, a polarization plate 2A bonded thereto and has, inside thereof, a number of color filter elements 3G, 3R and 3B arranged for the three primary colors: red (R), green (G) and blue (B) of a light. Further, between a pair of adjacent color filter elements 3 and 3, there is disposed a black matrix element or black stripe 4 that is designed to effect a glare protection and thus to obtain a clear color display. Of them, the black stripe is used exclusively for an electroplated color filter. Hence, numeral 4 here is used hereinafter in connection with the present invention to refer to a black matrix or black matrix element. Also, inside of the color filter 3 and the black matrix 4 mentioned above, there is disposed via a protective filter 5 a transparent electrode 6A.
On the other hand, outside of the other 1B of the above mentioned glass substrates, there is disposed via a polarization plate 2B a light illuminating plate 7 that functions to provide a back light. In the vicinity of an end surface of the said light illuminating plate 7, there is disposed a fluorescent lamp 8 that is adapted to cause the said light illuminating plate 7 to emit a light from over a surface thereof. Also, inside of the above mentioned glass substrate 1B, there are so arranged a plurality of gates 10 as are covered with an insulating layer 9. Further, inside of the said insulating layer 9, there are arranged hydrogenated amorphous thin film transistors 11, which are switching elements in opposition to the above mentioned gates 10, in a manner such that they may be surrounded with their respective protective members 12. And, inside of the above mentioned insulating layer 9, there are arranged transparent electrodes 6B, each associated with a said hydrogenated amorphous thin film transistor 11 for supplying the same with an electric current, respectively. It should also be noted that a liquid crystal LC is sealed in a tightly closed space between the above mentioned transparent electrodes 6A and 6B.
An explanation will now be given with respect the functions of the above mentioned black matrix 4 in a liquid crystal color display device as mentioned above. If the said black matrix 4 is held in the state in which the green color filter element 3G is turned ON and the red color filter element 3R and the blue color filter element 3B which are adjacent thereto are each turned OFF, it will function in a manner such that the back light which is surface emitted from the said light illuminating plate 7 may be transmitted only through the said green color filter 3G that is in the ON state and may not reach the said red and blue color filters 3R and 3B which are each in the OFF state.
And, the said black light matrix 4 is required to be composed of a material that is not only capable of shielding the back light but is of a low reflectivity to the back light. This is because if the green color filter element 3G is in the ON state and the red and blue color filter elements 3R and 3B which are adjacent thereto are each in the OFF state, when the black matrix 4 has a high reflectivity to the back light a said back light that is reflected at the black matrix 4 will be incident to those hydrogenated amorphous thin film transistors 11 which are each in the OFF state, corresponding to the red and blue color filter elements 3R and 3B, and may possibly turn those transistors 11 each into the ON state.
By the way, from the standpoint of the requirement for a lower reflectivity, a resinous black may be used as a composition for a black matrix 4 to provide a reflectivity that is as low as 0.5%, and is thus found to be much superior to a metallic chromium composition having a reflectivity of 50% and to a low reflection chromium composition having a reflectivity of 30% or less. A black matrix 4 that is composed of the resinous black has the problem, however, that its optical density (OD) which is an index that is indicative of a light shielding performance for the back light is 2.3 which is lower than 4 or more with the metallic chromium composition or the low reflection chromium. For this reason, a low reflection composition in which a chromium oxide component is mounted on a metallic chromium component is now becoming a leading composition for a black matrix 4.
An explanation will next be given with respect to a method of preparing a black matrix 4. In a method other than an electroplating method, a black matrix 4 is first formed on a glass substrate 1 and thereafter each of the color filter elements 3R, 3G and 3B are formed. While four methods of forming a color filter 3 have been employed up to present, a brief explanation will here be given of a pigment dispersion method and an electroplating method in the interest of convenience.
First, an explanation will be given of the pigment dispersion method.
Referring to FIG. 2, a glass substrate 1 on which a black matrix 4 is formed as shown at (a), is coated over its entire surface with a coloring resist 13 for a said red color filter element 3R, as shown at (b), by the spin coating method or the roll coating method. Then, as shown at (c), the above mentioned coloring resist 13 is coated with a photo resist 14 by the spin coating method or the roll coating method. Further, as shown at (d) and (e), the layers 13 and 14 through a photo mask 15 are exposed to a light and developed to form red color filter elements 3R.
Thereafter, by repeating a procedure as mentioned above with respect to a said green color filter element 3G and a said blue color filter element 3B, eventually the three color filter elements 3R, 3G and 3B are formed.
Next, an explanation will be given of the electroplating method.
Referring to FIG. 3, a glass substrate 1 has an ITO (indium tin oxide) thin film 16 for electroplating formed thereon by the evaporation method, whereafter the said ITO thin film 16 is coated with a photo resist 14, as shown at (a). Then, as shown at (b), (c) and (d), the layers 14 and 16 are exposed through a photo mask 15 to a light and developed, and etched to form a stripe of the said ITO thin film 16. Thereafter, as shown at (e), the glass plate 1 on which the said stripe of the ITO thin film 16 is formed is immersed in an electroplating or electro-depositing liquid in which particles of a pigment for a said red color filter element 3R are dispersed, and a positive voltage is applied between a pair of electrodes which are opposed to each other across the said ITO thin film 16 stripe so that those particles of the red pigment which have a particle size in the order of submicrons and are charged negative may electrolytically migrate towards the said ITO thin film 16 stripe charged positive and may then be electrolytically deposited on its surfaces. Subsequently, a said green color filter element 3G and a said blue color filter element 3B are electrolytically deposited by using respective electrolytic depositing or plating (i.e. electro-depositing or electroplating) liquids therefor.
In this manner, a color filter in a matrix 17 that comprises red, green and blue color filter elements 3R, 3G and 3B is formed on the said glass substrate 1 as shown at (f), referring continuingly to FIG. 3. Then, as shown at (g), a resinous black 18 having a photosensitivity is coated on the said color matrix 17 by the spin coating method or the roll coating method. Thereafter, if the said glass substrate 1 is exposed to a back light in the form of ultraviolet (UV) rays as shown at (h), the photosensitive resinous black 18 on the said colour matrix 17 will not be exposed to the said light in the presence of the intervening color filter elements 3R, 3G and 3B. And, it will be removed in the development process. As a consequence, a black matrix 4 will be formed in which the said resinous black 18 remains only between the adjacent ones of the said color filter elements 3R, 3G and 3B as shown at (i).
By the way, unlike a semiconductor IC a color filter 3 arrangement has an extremely large external size and requires a dimensional accuracy represented by a pixel pitch of 75 .mu.m and a gap of 25 .mu.m in the VGA (Video Graphic Array) specification, and a pixel of 70 .mu.m and a gap of 20 .mu.m in the SVGA (Super Video Graphic Array) specification and requires a positional accuracy per a panel of .+-.2 .mu.m. Hence, it has adopted its manufacturing process which relies much on a photo step, if the process were not the printing method.
And, it has been recognized that the printing method will be unable to meet with the requirement in the future that the mother glass should be much more large sized and the pixel should be made increasingly finer.
In forming a color filter arrangement according to a conventional method, there has also been a fear that due to the presence of a difference in level between a color filter 3 and a black matrix 4 arising from a difference between the film thicknesses, a problem of step coverage may be created at a location where there is such a difference in level. This has hitherto required the said difference in level to be filled up by coating a transparent resin over the entire surface and, if needed, the coated surface to be further processed by polishing, thus rendering the process much complicated.
The present invention has been provided in view of the foregoing problems taken into account, and has for its object to provide a method of manufacturing a color display, whereby a color filter and a black matrix therein can be manufactured while not using at all a photo-lithographic step that is complicated and yet that requires a high degree of cleanness or using the latter step at the minimum, the black matrix can be manufactured having a high optical density and a low reflectivity as well as an enhanced aperture rate, and an arrangement of columnar cell gap controlling spacers therein which are precisely positionable can be obtained.